Why Exergy?

By Professor Ibrahim Dincer

Thermodynamics

In the literature, thermodynamics is conventionally defined as the science of energy and entropy. Since energy and entropy do not have the same currencies (units) and represent two distinct entities, I personally define thermodynamics as the science of energy and exergy, which refers to the first- and second-law of thermodynamics.

This brings an advantage that we can easily assess and compare the performances under energy and exergy efficiencies correspondingly. When we analyse the systems, we need to write all four balance equations, including mass, energy, entropy and exergy balances for a complete and comprehensive solution.

Exergy vs Energy

In thermodynamics there are two potential laws for system analyses, namely first-law and second-law of thermodynamics. The first law is known as the conservation law which is defined as “Energy is neither created nor destroyed.

It just changes forms (for example, heat to work in a power plant and work to heat in a heat pump). The first law in this regard brings energy analysis. If we aim to define and identify irreversibilities, energy analysis becomes inadequate since it can neither quantify nor determine the system irreversibilities.

We need to go to the second law which is defined in various ways. In terms of entropy, it can be defined as “a law which represents the degree of disorder and states that entropy always increases. In terms of exergy, I define it as the law which brings up exergy as a potential tool to determine exergy destructions and losses and their true magnitudes and exact locations, and evaluate the actual performance through exergy efficiency.

It also states that exergy cannot be conserved, which can only be minimized if the measures are taken properly. A simple comparison of energy and exergy is illustrated in Figure 1. In Figure 1a, the illustration shows that a doctor uses a stethoscope to listen to heartbeats as energy does in thermodynamics. Figure 1b shows that the doctor holds the x-ray of the patient and can determine any problems as exergy can do for the systems.

Figure 1: An example of energy analysis.

Figure 2: An example of exergy analysis.

Exergy Conservation vs Energy Conservation

Energy conservation is utterly meaningless since energy analysis comes from the first law and fails to identify waste or the effective use of fuels and resources. If one aims for better use of resources, exergy conservation becomes a logical and meaningful target.

It is conceptually incorrect to aim to achieve “energy conservation” since energy is by default conserved under the first law. In fact, we intend to say exergy conservation. This clearly shows that we need to change the mentality and use the terms correctly and aim to achieve exergy conservation as illustrated in Figure 3.

As mentioned above, the first law is about energy conservation. Energy is always conserved (neither created nor destroyed). So, it also becomes utterly meaningless to discuss energy management since it is conserved.

In fact, we think of exergy management, but call it energy management. It is crucial to use the concepts correctly by saying and implementing exergy management mentality as illustrated in Figure 4. It implies that energy is fully managed by the first law whereas exergy can partially be managed by using the second law.

When we deal with processes, systems, applications, etc., there are four key items, namely analysis, improvement, design and assessment as one needs to address. In conjunction with this, we introduce a new AIDA concept to cover these four items. Aida in dictionary is a female name and originally comes from Arabic.

It has many meanings from being happy to distinguishing and from helper to visitor. Since these four letters make this name, we are inclined to take the meaning of helper and make the connection to exergy. Exergy is really a thermodynamic tool for analysis, improvement, design and assessment as clearly illustrated in Figure 4.

Exergy analysis permits many of the shortcomings of energy analysis to be overcome. Exergy analysis, by stemming from the Second Law of Thermodynamics, is useful in identifying the causes, locations and magnitudes of process inefficiencies.

The exergy associated with an energy quantity is a quantitative assessment of its usefulness or quality. Exergy analysis acknowledges that although energy cannot be created or destroyed, it can be degraded in quality, eventually reaching a state in which it is in complete equilibrium with the surroundings and hence of no further use for performing tasks.

Recently, exergy has been a primary tool, and its use has been extended to economy under exergoeconomics (or thermoeconomics) by including cost accounting; environment under exergoenviromics (or exergoenvironmental analysis) by including environmental impact accounting (assessment); and exergosustainability by including sustainability accounting (assessment).

Figure 5: An illustration of AIDA under exergy.

Exergy Efficiency vs Energy Efficiency

Energy (h) and exergy (y) efficiencies are often written for steady-state processes occurring in systems. In this regard, energy efficiency is defined as the ratio of useful energy output divided by the total energy input, and correspondingly, the exergy efficiency is defined as the ratio of useful exergy output divided by the total exergy input, as given below:

h = Useful energy output / Total energy input

and

y = Useful exergy output / Total exergy input

Exergy efficiencies often give more illuminating insights into process performance than energy efficiencies because (i) they weigh energy flows according to their exergy contents, and (ii) they separate inefficiencies into those associated with effluent losses and those due to irreversibilities. In general, exergy efficiencies provide a measure of potential for improvement.

Closing Remarks

In this article, the advantages of applying exergy analysis in place of or in concert with energy analysis are explained and illustrated. It is increasingly important to enhance the use of exergy to other disciplines and develop an exergy mentality to achieve better design and analysis, better efficiency, better cost effectiveness, better use of resources, better environment, better energy security and better sustainability.

For further information and details, the following references may be beneficial:

About Professor Ibrahim Dincer

Ibrahim Dincer is a full professor of Mechanical Engineering in the Faculty of Engineering and Applied Science at UOIT. He is Vice President for Strategy in International Association for Hydrogen Energy (IAHE) and Vice-President for World Society of Sustainable Energy Technologies (WSSET).

Renowned for his pioneering works in the area of sustainable energy technologies he has authored and co-authored numerous books and book chapters, more than 900 refereed journal and conference papers, and many technical reports. He has chaired many national and international conferences, symposia, workshops and technical meetings. He has delivered more than 250 keynote and invited lectures. He is an active member of various international scientific organizations and societies, and serves as editor-in-chief (for International Journal of Energy Research by Wiley; International Journal of Exergy, International Journal of Global Warming and International Journal of Research, Innovation and Commercialism by Inderscience; and The Open Environmental Engineering Journal by Bentham), associate editor, regional editor, and editorial board member on various prestigious international journals.

He is a recipient of several research, teaching and service awards, including the Premier’s research excellence award in Ontario, Canada in 2004. He has made innovative contributions to the understanding and development of sustainable energy technologies and their implementation. He has actively been working in the areas of hydrogen and fuel cell technologies, and his group has developed various novel technologies/methods/etc.

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Date Added: Jul 22, 2013
| Updated: Jul 21, 2015

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